In a vehicle having a De-NOX catalytic post-processing device (for reducing nitrogen oxides) (hereinafter, referred to as a NOX purification device) such as a hydrocarbon selective catalytic reduction (HC-SCR) device or a lean NOX trap (LNT) device in an exhaust gas flow path of an engine, a NOX sensor may be installed in both upstream and downstream sides of the NOX purification device, and a NOX conversion efficiency can be computed based on a NOX concentration ratio detected by such a NOX sensor.
It is desirable to obtain a temporal change of the NOX conversion efficiency and perform on-board diagnostics (OBD) for determining that the NOX purification device has failed when the NOX conversion efficiency is degraded over a threshold value.